Preservation of rubber



Patented Dec. 5, 1944 PRESERVATION OF RUBBER David J. Beaver, St.Albans, W. Va., assignor to Monsanto Chemical Company, St. Louis, Mo., a

corporation of Delaware No Drawing. Application 'April 15, 1942, SerialNo. 439,127

22 Claims. (01. 260-810) The present invention relates to the art ofrubber manufacture, and particularly relates to rubber compositionswhich resist deterioration due to aging or to exposure to theatmosphere. It has long been known that such deterioration can begreatly retarded by treating the rubber before or after vulcanizationwith certain substances known as age-resistors or antioxidants. Oneobject of this invention is to provide a superior class of antioxidantsfor rubber.

The age resistingcharacteristics of a vulcanized rubber product can bereadily'ascertained by subjecting samples of the vulcanized product in abomb to the action of air or oxygen under elevated pressure. The agedrubber samples are then examined and tested and the test data comparedwith the results obtained on testing the unaged rubber samples. Thedeterioration in properties effected as a result of the oxidation isindicative of the result that would normally be rubber depending uponthe 'conditions of the test.

The stabilizing agents of the present invention belong to, the class ofhydroxy substituted aromatic thio ethers also known as phenol sulfides.filed May 27, 1941, of which the present application is acontinuation-in-part reveals that sulfides of mono hydric dialkylphenols in which at least one of the alkyl groups contains more than twocarbon atoms are valuable non-discoloring antioxidants. In accordancewith the present invention it has been discovered that the compoundsbelonging to this class of phenolic sulfides the sulfides of dialkylmono-hydric phenols in which at least two of the positions ortho andpara to the hydroxy group are unsubstituted except by sulfur comprise aparticularly efiicacious group and are markedly superior to sulfidesderived from dialkyl phenols wherein only one or not any of thesepositions are unoccupied or where both the alkyl groups contain lessthan three carbon atoms or where only one alkyl group is present.

My prior application Serial No. 395,445,

Typical examples of compounds coming within the purview of thisinvention comprise di(1- hydroxy 3-methyl '6-isopropyl phenyl)monosulfide and. disulfide; di(1-hydroxy 3"-methyl 6-tertiary butylphenyl) monosulfide and disulfide,

phenyl) monosulfide and disulfide, di(1hydroxy 2-methyl 5-tertiary butylphenyl) monosulfide and disulfide, di(1-hydroxy Z-methyl S-tertiary Iii)"amyl phenyl) monosulfide and disulfide, di(1- hydroxy 3-methyl4-tertiary butyl phenyl) monosulfide and disulfide, di(1-hydroxy3-methyl 4- isopropyl phenyl) monosulfide and disulfide, di(lhydroxy3-methyl 4-tertiary amyl phenyl) monosulfide and disulfide, di(1-hydroxy3-tertiary butyl 4 methyl phenyl) monosulfide and disulfide,di(1--hydroxy 3-isopropyl 4 methyl phenyl) mono sulfide and disulfide,di(l-hydroxy 3,6-clitertiary butyl phenyl) monosulfide and disulfide,di(1- hydroxy 2,5 ditertiary butyl phenyl) monosulfid'e and disulfide,di(1-hydroxy 2-methyl 3 isopropyl phenyl) monosulfide and disulfide,di(1- hydroxy 2-methy1 3-tertiary butyl phenyl) monosulfide anddisulfide, di(1-hydroxy 3,5 di tertiary butyl phenyl) monosulfide anddisulfide, di(1- hydroxy 3,5 di tertiary amyl phenyl) monosulfide anddisulfide, di(l-hydroxy 3-methyl 5-isopropyl phenyl) monosulfide anddisulfide, di(1-hydroxy ,3-methyl 5-tertiary butyl phenyl) monosulfideand disulfide, di(1-hydroxy 3-methyl 5-tertiary amyl phenyl) monosulfideand .disulfide, di(1-hydroxvy 3-methyl 4 secondary butyl phenyl)monosulfide and disulfide, di(1-hydroxy S-methyl 4 isoamyl phenyl)monosulfide and disulfide and equivalents/and analogues thereof. It issignificant that di(1-hydroxy 2 methyl 4 tertiary butyl phenyl)disulfide was found not to possess the superior antioxidant propertiescharacteristic of the preferred class.

Since the preferred age resisters are advantageously employed in whiterubber goods, constituents which discolor rubber as for example aminoand other strongly basic constituents should be absent. Strongly acidicgroups afiect' the cure but a halogen substituent may be present.

Members of the preferred class of age resisters can be obtained by theaction of sulfur monochloride (S2C12) or sulfur dichloride (SClz) on thedialkyl aromatic hydroxy compound. Where'convenient ondesirable othermethods of synthesis may be used, and it is to be understood that .thisinvention is not limited to any method of synthesis but pertains broadlyto'the use of the materials as age resisters. However, it may be pointedout that since more than one position at which the sulfur may attach tothe ring isavailable, more than a single reaction product may be formed,and it is within the purview of this invention to employ the compositereaction products directly as antioxidants. For example thymol andcarvacrol react readily with sulfur dichloride in an organicsolvent toproduce crystalline solids M. P. (purified) 152-153 C. and 17l-172 C.respectively and minor amounts of other material which remains in theorganic solvent. If the solvent is evaporated off without separating thecrystalline solid, a composite resinous reaction product remains whichpossesses nearly the same antioxidant strength as the crystalline solidand has substantially no discoloring effect on the cured stocks.Similarly, 4-tertiary butyl meta cresol reacts with sulfur dichloride inether solution producing a crystalline solid melting at 157-158" C. andan even larger proportion of resinous reaction product. Again thecomposite reaction product was found to possess nearly the sameantioxidant strength as the pure crystalline compound and did notdiscolor the rubber.

A method of preparation which was found to give good results comprisesadding a solution of sulfur monochloride or dichloride in carbontetrachloride or other suitable solvent to a rapidly stirred solution ofthe phenol in the same solvent. The reactions are usually instantaneousand take place with little evolution of heat. The mixture is then cooledto precipitate out the crystalline material and filtered, or where theproduct is a liquid or where it is desired to isolate the compositereaction product, the solvent is simply removed by evaporation or othersuitable means. Products having antioxidant properties were obtained,for example, by the interaction of sulfur dichloride and 1,3,5 xylenol,ptertiary amyl phenol, p-chlor m-cresol, 3,5 dimethyl 4 chlor phenol,o-benzyl phenol, p-chlor phenol, 8 hydroxy quinoline, the mono benzylether of catechol, p-chlor o-cresol and 3 methyl 4,6 tertiary butylphenol.

As specific embodiments of the invention rubber stocks containingtypical members of the preferred class of preservatives were compoundedfrom a white rubber base mix,

The rubber mixtures were then vulcanized in a press at the temperatureof twenty pounds steam pressure per square inch and artifically aged byheating in a bomb under 80 pounds air pressure per square inch and at250 F. The tensile strength of the cured rubber products before andafter aging are set forth below:

Table I Cure H" Tensile Ultimate Stock time in a d strength elongation,

mins. 38 in lbs/in. percent Base mix 30" 4, 400 685 A so 0 4, 370 700 300 4, 250 010 30 s 560 20s s0 0 2,490 555 30 0 2,450 045 45 0 4,180 66545 0 4, 010 670 45 0 4,175 050 45 0 875 480 45 s 2, 550 580 45 0 2, 400575 00 0 4,210 660 so 0 4, 400 685 00 0 4,200 660 00 e 755 400 60 6 2,485 575 00 0 2,300 560 The above data show the superior age resistingproperties imparted to rubber by the preferred class of materials andillustrate the enormous improvement over a stock containing noantioxidant. In addition samples of the cured stocks were exposed to asunlamp for 74 hours. At the end of the exposure only a slightdiscoloration of the stocks containing an antioxidant had taken place.

As further specific embodiments of the invention illustrating thesuperiorv preservative action of the new and improved class ofcompounds, other stocks were prepared by adding 1.0 part by 'weight of aphenol sulfide to the base mix described above. The stocks so compoundedwere vulcanized in the usual manner at the temperature of twenty poundssteam pressure Der square inch and artiflcally aged by heating for fourhours in a bomb under eighty pounds air pressure per square inch and at250 F. The tensile strengths of the aged rubber products were determinedin the usual manner and the superiority over the base mix noted. Forconvenience the tensile strength 0! the base mix, i. e. the stockcontaining no antioxidant,was arbitrarily taken as 100 and the actualtensiles oi. the stocks containing a phenol sulfide divided by theactual tensiles of the base stock and multiplied by 100 to give acomparative rating. The figures given below are therefore ratios of thetensile strength at break of the stocks containing an antioxidant tothat of the same stock without an antioxidant. They set forth directlythe extent of the protection provided by the antioxidant.

Table II Ratio oi aged tensiles 1.11-

Antioxidmt 00 min. cures 30 min. cures min. cures 40 The superiority ofhydroxy aromatic sulfides prepared from dialkyl mono hydric phenols inwhich two of the three positions para and ortho to the hydroxy group areunoccupied is clearly shown by the above data.

Obviously practice of this invention is not limited to the specificcompositions given above, such compositions being merely illustrative ofthe manner of employing the antioxidants or age resisters 0! thisinvention. The antioxidants or age resisters may be employed inconjunction with other vulcanizing agents than those specificallydisclosed for this invention is applicable generally to pure'rubber orrubber compositions of the most varied nature. Furthermore the preierredclass oi. materials may be employed in conjunction with otheraccelerators than those specifically shown with varying differences intensile and modulus properties but still exhibiting the desirableproperties of the class.

It is to be understood that the preferred class of materials may beincorporated into the rubber by milling or similar process, or may beadded to the rubber latex before its coagulation, or applied to thesurface of a mass of crude r vulcanized rubber and that the termtreating" as employed in the claims is used in this generic sense.

The term a rubber is employed in the claims to define a sulfurvulcanizable plastic material which possesses high extensibility underload coupled with the property of forcibly retracting to approximatelyits original size and shape after w the load is removed. Such productsinclude for example natural rubber including reclaimed rubber, balataand gutta percha and synthetically prepared rubbers as for exampleHycar, Perbunan and Buna S all of which are said to be polymers orco-polymers of butadiene. These and other natural or syntheticallyprepared sulfur vulcanizable products which deteriorate upon aging areincluded whether or not admixed with fillers, pigments or acceleratingagents.

The present invention is limited solely by the claims attached hereto aspart of the present specification.

What is claimed is:

1. The method ,of preserving a rubber which comprises treating a rubberwith a sulfide of a monohydric dialkyl phenol in which a carbon atom ofeach of two nuclei of the phenol are joined together by less than threesulfur atoms and the alkyl groups contain less than six carbon 3 atomsand at least one alkyl group is a branched chain alkyl group and whichis unsubstituted, except for sulfur, in two of the positions ortho andpara to the hydroxy group.

2. The method of preserving a i'ubber which comprises treating a rubberwith a mono. sulfide of an alkyl cresol in which said alkyl group .is abranched chain containing less than six carbon atoms and which isunsubstituted, except for sulfur, in two of the positions ortho and parato o the hydroxy group. i

3. The method of preserving a rubber which comprises treating a rubberwith a monosulfide of a tertiary alkyl cresol which is unsubstituted,except for sulfur, in two of the positions ortho '45 and para to thehydroxy group.

4. The method of preserving a rubber which comprises treating a rubberwith did-hydroxy 2-methyl 5-isopropyl phenyl) monosulfide.

5. The method of preserving a rubber which comprises treating a rubberwith di(1-hydroxy S-methyl 4-tertiary butyl phenyl) monosulfide.

6. The method of preserving natural rubber which comprises treatingnatural rubber with a sulfide of a monohydric dialkyl phenol in which acarbon atom of each of two nuclei of the phenol are joined together byless than three sulfur atoms and the alkyl groups contain less than sixcarbon atoms and at least one alkyl group is a branched chain alkylgroup and which is unsubstituted, except for sulfur, in two of thepositions ortho and para to the hydroxy group.

7. The method of preserving natural rubber which comprises treatingnatural rubber with di- (l-hydroxy Z-methyl 5-isopropyl phenyl) mono: 6sulfide.

8.The method of preserving natural rubber which comprises treatingnatural rubber with di(1-hydroxy 3-methyl 4-tertiary butyl phenyl)monosulfide.

9. Thevulcanized rubber product obtained by heating a rubber and sulfurin the presence of a sulfide of a monohydric dialkyl phenol in which acarbon atom of each of two nuclei of the phenol are joined together byless than three sulfur atoms and the alkyl groups contain less than sixcarbon atoms and at least one alkyl group is a branched chain alkylgroup and which is unsubstituted, except for sulfur, in two of thepositions ortho and para to the hydroxy group.

10. The vulcanized rubber product obtained by heating a rubber andsulfur in the presence of a mono sulfide of an alkyl cresol in whichsaid alkyl group is a branched chain containing less than six carbonatoms and which is unsubstituted, except for sulfur, in two of thepositions ortho and para tothe hydroxy group.

11. The vulcanized rubber product obtained by heating a rubber andsulfur in the presence of a monosulfide of a tertiary alkyl cresol whichis unsubstituted, except for sulfur, in two of the positions ortho andpara to the hydroxy group.

12. The vulcanized rubber product obtained by heating a rubber andsulfur in the presence of di(1-hydroxy '2-methyl fi-isopropyl phenyl)monosulfide.

13. The vulcanized rubber product obtained by heating a rubber andsulfur in the presence of di(1-hydroxy 3-methyl 4-tertiary butyl phenyl)monosulfide.

14. The vulcanized rubber product obtained by heating natural rubber andsulfur in the presence of a sulfide of a monohydric dialkyl phenol inwhich a carbon atom of each of two nuclei of the phenol are joinedtogether by less than three sulfur atoms and the alkyl groups containless than six carbon atoms and at least one alkyl group is a branchedchain alkyl group and which is unsubstituted, except for sulfur, in twoof the positions ortho and para to the hydroxy group.

15. The vulcanized rubber product obtained by heating natural rubber andsulfur in the presence of did-hydroxy 2-methyl 5-isopropyl phenyl)monosulfide.

16. The vulcanized rubber product obtained by heating natural rubber andsulfur in the presence of di(1-hydroxy 3-methyl 4-tertiary butyl phenyl)monosulfide.

17. The method of preserving a rubber which comprises treating a rubberwith a sulfide of a monohydric dialkyl phenol possessing the structurewhere R and R are alkyl groups of less'than six carbon atoms at leastone alkyl group on each nucleus is a branched chain alkyl group and n isan integer less than three.

18. The method of preserving a rubber which comprises treating a rubberwith a sulfide of an alkyl cresol possessing the structure CH3 CH3 whereR is a branched chain alkyl radical containing less than six carbonatoms and n is an integer less than three.

19. The method of preserving a rubber which comprises treating a rubberwith di(1-hydroxy 3-methyl fi-isopropyl phenyl) monosulfide.

20. The vulcanized rubber product obtained by heating a rubber andsulfur in the presence of a sulfide of a monohydric dialkyl phenolpossessing the structure where R and R are alkyl groups of less than sixcarbon atoms at least one alkyl group on each nucleus is a branchedchain alkyl group and n 1 is an integer less than three.

21. The vulcanized rubber product obtained by heating a rubber andsulfur in the presence of a sulfide of an alkyl cresol possessing thestructure where R, is a branched chain alkyl radical contaming less thansix carbon atoms and n is an integer less than three.

22. 'I'he vulcanized rubber product obtained by heating a rubber andsulfur in the presence of di- (l-hydroxy 3-methyl fi-lsopropyl phenyl)monosulfide.

DAVID J. BEAVER.

